The present invention relates to optical data transmission and more particularly to a method and apparatus for calibrating a laser diode for FSK (Frequency Shift Keyed) data communications.
In order to employ a laser diode for FSK data communication, it is important that the laser diode be calibrated so that, during communications, it will operate at the desired power level, at the desired nominal wavelength, and at the desired modulation deviation. Precise control of these parameters is particularly necessary where the laser diode transmitter will be employed in a free space data communications system in which detection will be by means of optical heterodyning. In such a situation, the nominal laser wavelength must be held within known tolerances to insure that the intermediate frequency (i.f.) signal obtained after optical mixing with a local oscillator lies within the necessarily limited receiver bandwidth. Similarly, the frequency shift tone spacing must be held within limits to insure that the transmitted tones lie within the centers of the matched filters in the receiver. It is also desirable to control optical power within reasonable limits so as to maintain the data link margin. As is well known in the art, however, the characteristics of typical laser diodes are not necessarily stable, and change as the diode ages as well as with various environmental parameters.
Among the several objects of the present invention may be noted the provision of a method and apparatus for calibrating laser diodes for FSK data communications; the provision of such apparatus which will adjust the nominal laser diode wavelength to a precise standard; the provision of such apparatus which will permit automatic recalibration of a laser diode and diagnosis of its characteristics; the provision of such apparatus which will adjust FSK tone spacing to a precise standard; the provision of such apparatus which can be constructed in a very compact form suitable for inclusion in a satellite communications system; the provision of such apparatus which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.